1. Field of the Invention
The present invention relates to a floating offshore structure and more particularly to a structure floating offshore and moored so as to be positioned at a substantially fixed position, such as a floating deck structure usable for submarine excavation, for example.
2. Description of the Prior Art
With a steep rise in demand for oil, submarine oil fields have been vigorously developed. Also, other submarine resources have been energetically investigated and developed. As a movable excavating apparatus for effecting these developments, for example, a floating offshore structure is employed. The floating offshore structure of this type is a large-sized structure having a diameter or longitudinal or lateral dimension of nearly 100 m, for example, and a displacement of nearly from 30 to 40 thousand tons. Such a structure employes a large number (e.g., 12) of mooring hawsers so as to be moored at a fixed position on the sea by means of anchors connected to the ends of these mooring hawsers respectively.
The floating offshore structure of this type which is employed in an icy sea area in cold waters where floating ice is present conventionally has a planar shape which is a circle or a polygon close to a circle. Therefore, in case of conducting excavation in a frozen sea, this structure is made to have no directional property with respect to the floating ice force (the collision force of floating ice) and, hence, is independent of the flowing direction of the floating ice. On the other hand, however, since the structure has a circular section, the width allowing floating ice to collide is large relatively for a given effective deck area, so that the structure receives a correspondingly large collision force of floating ice. For the same reason, the movement of the structure due to waves is larger.
Moreover, in a floating offshore structure for submarine excavation, such as an oil rig, a vertical through-hole is formed in the structure body, i.e., the hull, and an excavating drill pipe driven by an excavator on the deck is passed through the through-hole and extended to the sea bottom to carry out operations. In the floating offshore structure of this kind, the vertical through-hole has hitherto been formed as a hole opened in the hull bottom. Therefore, in case of using the structure in a frozen sea, such as cold waters where floating ice is present, some of the blocks of ice broken by the outer wall of the hull on the fore side (the side with which floating ice collides) scatter in the sea around the hull bottom and may undesirably enter the vertical through-hole from its opening formed in the hull bottom, resulting in damage to the excavating drill pipe and mooring hawsers extending through the through-hole.